Engine control systems often determine the amount of fuel to inject by measuring a manifold pressure, along with other engine operating conditions. This method is often referred to by those skilled in the art as the speed density method. In this method, a mean value model of engine operation is constructed, where an average manifold pressure at a given speed results in a certain air flow into the cylinder. In this type of system, measurement of the manifold pressure is critical for proper prediction of the air flow into the cylinder and thus for proper air/fuel ratio control.
As stated above, many methods are available to estimate cylinder air charge using a manifold pressure sensor. Typically, engine maps are provided that provide a cylinder air charge as a function of measured manifold pressure, manifold temperature, and engine speed. In engines that also utilize exhaust gas recirculation, an improved cylinder air charge estimate is obtained by providing adjustments based on the amount of exhaust gas recirculation.
One particular method is described in U.S. Pat. No. 5,205,260. In this method, an EGR flow is estimated based on differential pressure across a flow control valve and based on a cross-sectional area of the valve. Then, this flow is used in a manifold filling model to estimate a partial pressure of EGR in the intake manifold. Then, based on this partial pressure of EGR and measured manifold pressure, a cylinder air charge value is computed.
The inventors herein have recognized a disadvantage with the above system. In particular, estimating EGR flow in this manner leads to estimation inaccuracies, due to valve area uncertainty. This uncertainty may be caused by deposits in the valve. Since inaccuracies in EGR flow directly affect estimated cylinder air charge, this leads to inaccuracies in calculated fuel injection amount and therefore may degrade air-fuel ratio control.
Another approach to determining EGR amount has used a differential pressure measurement across an orifice to infer a flow of exhaust gas. Traditionally, the orifice is located upstream of the exhaust gas recirculation flow control valve. Thus, the pressure measurements are shielded from the intake manifold pressure pulsations; however, the pressure measurements are not shielded from the exhaust pressure pulsations. In the traditional system, the high frequency pressure pulsations present in the pressure measurements are reduced by using a conventional low pass filter. Such a system is disclosed in U.S. Pat. No. 5,613,479.
The inventors herein have recognized a significant opportunity to reduce total system cost by relocating the orifice downstream of the exhaust gas recirculation flow control valve but before the intake manifold. Thus, the manifold pressure sensor can be used to measure the pressure downstream of the orifice and a single absolute pressure sensor can be used to measure the pressure upstream of the orifice. This creates the needed differential pressure to measure exhaust gas recirculation flow, as well as an opportunity to estimate cylinder fresh charge.